The present invention relates to control of a pressure swing adsorption system; and, more particularly, to a method and apparatus for controlling product repressurization in a multi-bed system to improve system reliability.
Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multicomponent gas stream containing at least two gases having different adsorption characteristics. The more-strongly adsorbable gas can be an impurity which is removed from the less-strongly adsorbable gas which is taken off as product; or, the more-strongly adsorbable gas can be the desired product, which is separated from the less-strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more-strongly adsorbable gases, such as ethylene, from a feed to produce an ethylene-rich product.
In pressure swing adsorption, a multicomponent gas is typically fed to at least one of a plurality of adsorption beds at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, feed to the adsorber is terminated and the bed is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less-strongly adsorbed component or components remaining in the bed to be drawn off without significant concentration of the more-strongly adsorbed components. Then, the bed is depressurized by a countercurrent depressurization step wherein the pressure on the bed is further reduced by withdrawing desorbed gas countercurrently to the direction of feed. Finally, the bed is purged and repressurized. The final stage of repressurization is with product gas and is often referred to as product repressurization.
In multi-bed systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. Nos. 3,176,444 to Kiyonaga, 3,986,849 to Fuderer et al, and 3,430,418 and 3,703,068 to Wagner, among others, describe multi-bed, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties.
Every adsorber in a multi-bed system (PSA unit) is equipped with a plurality of valves operated by a cycle controller. In addition to feed, product, and waste gas valves, other valves are typically employed to enable pressure equalizations between adsorbers. Pressure swing adsorption in multi-bed systems remains essentially a batch process, simply cycling a number of beds through a coordinated series of steps, and a number of valves are idle at any given time.
Typically, there are valves in a PSA unit which, if they fail, can cause shutdown of the entire unit. In U.S. Pat. No. 4,234,322, De Meyer et al describe a PSA unit having at least eight phase staggered operated beds which permits continued operation even where one bed must be removed due to valve failure. However, one valve (namely, 101) is employed in product repressurization of all of the nine beds shown in FIG. 1. Thus, should this valve fail, the whole unit would have to be shut down.
There is a present need for a scheme which would permit rearranging flow in a pressure swing adsorption system to eliminate one or more valves, especially where this is consistent with the need to maintain a substantially constant flow of product from the system. Moreover, it would be especially desirable to have an improved system which eliminated system-dependent valves, which, if they failed, could cause shutdown of all beds of a multi-bed system.